In a principal aspect, the present invention relates to a steel alloy and a process for making such an alloy which exhibits new levels of strength and toughness while meeting processability requirements. The ultratough, weldable secondary hardened plate steel alloys for structural applications exhibits fracture toughness (KId 200 ksi.in1/2) at strength levels of 150-180 ksi yield strength, is weldable and formable.
Throughout the history of materials development, there has been an ever-increasing need for stronger, tougher, more fracture resistant and easily weldable plate steels for structural applications at minimal cost. Unfortunately, however, any increase in strength is rarely achieved without concomitant decreases in toughness and ductility, which limits the utility of most ultrahigh-strength steels. The best combinations of strength and toughness have usually been obtained from martensitic microstructures as shown in FIG. 1.
High strength bainitic steels have not been as successful in practice because of coarse cementite particles in bainite that are detrimental to toughness. Nonetheless, a potential benefit motivating research of air-hardened steels containing bainite/martensite mixtures is the ease of processing, which may lead to a product with good performance at a relatively lower cost. The possibility of improving the strength and toughness simultaneously using fine-grained bainitic ferrite plates and enhancing the toughness by transformation toughening effects presents a technological challange. Further improvements of strength can possibly be achieved with co-precipitation of alloy carbides and bcc copper for easily weldable, low-carbon steels again presenting a technological challenge.
It is now known that the interaction of deformation-induced martensitic transformation of dispersed austenite with fracture-controlling processes such as microvoid induced shear localization results in substantial improvements in fracture toughness called Dispersed Phase Transformation Toughening (DPTT). Transformation toughening is attributed to modification of the constitutive behavior of the matrix through pressure-sensitive strain hardening associated with the transformation volume change. The transformation behavior and the toughening effects are controlled by the stability of the austenite dispersion. For transformation toughening at high strength levels, the required stability of the austenite dispersion is quite high and can be achieved only by size refinement and compositional enrichment of the austenite particles. The size influences the characteristic potency of nucleation sites in the particles while the composition influences the chemical driving force and interfacial friction for the martensitic transformation. The size refinement and the compositional enrichment of the austenite can possibly be controlled with heat treatments such as multi-step tempering.
With this general background, design objectives motivating the invention are the achievement of extreme impact fracture toughness (Cv>85 ft-lbs corresponding to fracture toughness, KId>200 ksi.in1/2 and KIc>250 ksi.in1/2) at high strength levels of 150-180 ksi yield strength in weldable, formable plate steels with high resistance to hydrogen stress corrosion cracking (KISCC/KIC>0.5). Design goals are marked by the star in the cross-plot of KIc fracture toughness and yield strength illustrated in FIG. 2. This design aims to substantially expand the envelope marked as “steels” to the top right corner of the plot. Optimization of such a system and achievement of design goals can possibly be effectively achieved by consideration of the methods of systems design. FIG. 3 describes in general a system approach to design steel with the specified strength, toughness levels as well as optimum weldability and hydrogen resistance.
As further background, recent studies have shown that selection of fine Ti(C,N) as a grain refining dispersion contributes to increasing the fracture resistance by delaying the coalescence of microvoids among the primary voids. Studies have also suggested that the resistance to primary void formation and coalescence is proportional to inclusion spacing. Thus, it may be desirable to reduce the volume fraction of primary inclusions or coarsen inclusions for a given volume fraction. This can be achieved by clean melt practices and tight composition control. However, engineering design fracture toughness parameters like KIc and KId are difficult and expensive to measure. Thus for preliminary design analyses, small-scale inexpensive fracture measurements like Charpy V-notch impact energy (CV) values may be used to estimate KIc and KId. Studies of fracture toughness dependence on loading rate measured over a temperature range have shown that KIc fracture toughness values under static and intermediate loading are about 20% higher than the KId measured under impact loading. An approximate correlation between KIc and CV test results for conventionally grain-refined steels is as follows:KIC2=ACV  (1)where A is a constant of proportionality. Fitting equation (1) to results from high Ni steels is shown in FIG. 4.
According to these relationships, the CV impact toughness objective of 85 ft-lbs corresponds to a KIc fracture toughness under static loading of 250 ksi.in1/2 and a dynamic KId of 200 ksi.in1/2.
A fine carbide dispersion may need to be obtained in order to achieve the desired strength level. Coherent M2C carbides have been used in secondary hardened steels that are currently in use. Previous work to optimize the carbide particle size for maximizing the strength 3 nm carbide precipitates corresponding to the transition from particle shear to Orowan bypass may provide maximum strength. Thermodynamics and kinetics of carbide precipitation may need to be controlled to obtain such a fine M2C carbide dispersion. The driving force for M2C nucleation may also be maximized by proper control of the amount and ratio of carbide formers in the alloy to refine the M2C particle size. Sufficient M2C precipitation may need to be achieved to dissolve cementite in order to attain the desired toughness levels because coarse cementite particles are extremely deleterious as microvoid nucleation sites. Tempering times should also be minimized to prevent impurity segregation at grain boundaries.
Even if low alloy carbon levels are maintained, steels containing higher alloying content might help in achieving the desired combination of mechanical properties, but may reduce the weldability of the material by increasing hardenability. For any structural material, the heat-affected zones (HAZ) adjacent to the welded joints are considered to be the weakest links. Weldability of steels is generally controlled by both the matrix and the strengthening dispersion structures. As a rule of thumb, for adequate weldability of the steel C content of the alloy should be kept below 0.05 wt %. This in turn limits the C available for M2C strengthening. For a bainitic matrix, modification of the hardenability of the steel may provide bainite with a much lower cooling rate. However, weldability can deteriorate as the hardenability increases. Again, numerous interrelated technological challenges are apparent in view of various known considerations.
Ultra-high strength steels are prone to a decrease of fracture toughness in aqueous environments due to hydrogen assisted cracking. This reduction of toughness is caused by intergranular brittle fracture associated with impurity segregation to grain boundaries, which may reduce toughness of the steel by as much as 80% in a corrosive environment. The common impurities in steel are P and S, both of which are embrittlers since they have lower free energy on a surface than at a grain boundary. An effective way of reducing them is by cleaner processing techniques or impurity gettering. Impurity gettering can tie up P and S as stable compounds formed during solidification. La and Zr have been found to be effective impurity gettering elements. Another approach to minimize impurity effects is by design of grain boundary chemistry. Segregating elements like W and Re preferentially on the grain boundaries that may enhance grain boundary cohesion could be beneficial to the stress corrosion cracking resistance. Small amounts of dissolved B may also help in grain boundary cohesion. In view of the numerous foregoing factors and information, a need for an improved high strength plate steel was addressed.